In conventional electrorefining of copper, a copper anode made of about 99.8% copper is placed in an electrolyte, typically copper sulphate/sulphuric acid. By the action of current applied to the anode, the copper dissolves and is deposited at a cathode to a purity level of at least 99.99%. Such electrorefining produces slimes that precipitate at the bottom of the electrorefining cell, and can be removed from the electrolyte through conventional liquid/solid separation. The slimes contain precious metals, like gold, silver, platinum, etc. originally in the anode, as well as small residues of copper. In addition, they contain undesirable impurities such as arsenic, tellurium, selenium, bismuth etc.
Because of the high value of their content, electrorefiners treat the slimes to recover the precious metals. Several treatment methods have been proposed for recovering the various metals and non-metals from the slimes.
Copper and lead electrorefining slimes are a major source of tellurium. Tellurium is a nuisance element that must be removed to recover precious metals from copper slimes. For the overall slime treatment process efficiency, it is important to extract as much tellurium as possible. Such extraction is typically carried out in autoclaves. Any tellurium not removed in the autoclave goes to the top blown reverbatory converter (TBRC) where it is captured in the slag. This slag is recycled, as flotation tailings, to the smelter creating additional, unwanted, tellurium input to the smelter.
Several methods have been proposed to recover tellurium from electrorefining slimes. For example, in U.S. Pat No. 2,076,738, there is disclosed a method comprising fluxing the slimes with soda ash or caustic soda to produce a slag that is subsequently cooled and leached with hot water to extract sodium tellurite.
U.S. Pat No. 2,981,595 teaches a process for recovering tellurium from anode slimes produced during copper electrorefining. The process comprises digesting the slimes in sulphuric acid to form soluble tellurium sulphate, and the solid fraction is removed by filtration. Metallic copper is then added to the filtrate to form a copper-tellurium cementation product containing finely divided tellurium particles. Some metallic silver is also precipitated with the tellurium. Metallic tellurium is then recovered from the cementation product through a smelting operation.
U.S. Pat. No. 3,127,244 discloses a process for separately recovering selenium and tellurium having an oxidation state of from +2 to -2 from copper electrorefining slimes. The process uses pressure leach in an autoclave in the presence of an excess of alkali metal hydroxide, and oxygen.
U.S. Pat. No. 4,047,939 is concerned with a process for the leaching of copper electrorefining slimes with dilute sulphuric acid under oxygen partial pressure at a temperature between 80 and 160.degree. C. to dissolve substantially all copper and tellurium.
After a liquid-solid separation, the tellurium in the filtrate is cemented with metallic copper as copper telluride.
In a presentation made by Decker et al. at the AIME in Las Vegas in 1976, it is observed that if the molar ratio of Ag/Se+Te in the slimes is greater than 2.8, then silver leaches first. On the other hand, if the Ag/Se+Te molar ratio is smaller than 2.8, tellurium leaches first. The paper suggests that the anode furnace charges should be conditioned so that the ratio silver/selenium+tellurium in the anodes is higher than 2.8, the goal in this paper being to leach the silver first whenever possible. However, the suggested conditioning of the charges in the anode furnace is not always practical or possible because it would require tight control of the feed material. Further, the composition of the feed materials may not allow for the proper ratio to be obtained.
U.S. Pat. No. 4,299,810 describes a process for separating selenium and tellurium from copper slimes. Initially, the slimes are leached with concentrated sulphuric acid to dissolve selenium and tellurium. Leaching is further pursued at high temperature to precipitate the dissolved tellurium. After a solid-liquid separation, the solid is leached with water to solubilize the tellurium.
U.S. Pat. No. 5,160,588 teaches a process for removing tellurium from copper slimes, wherein a copper electrode plate is suspended in a bath containing a solution of copper and tellurium sulphate, for precipitating copper telluride thereon.
U.S. Pat. No. 5,271,909 is concerned with an improved method for extracting tellurium from copper slimes. The method comprises leaching the slimes in an autoclave at a temperature of from 100 to 200.degree. C. to dissolve copper, tellurium and nickel, halting the leaching process and adding a reducing agent to reprecipitate excess of silver and/or selenium dissolved in the solution. Although the high temperature and oxygen pressure allows a higher extraction yield of tellurium, it has the major disadvantage of causing excessive leaching of silver and selenium, and an additional reduction step is therefore required to precipitate the latter two elements.
As it can be seen from the above, although several methods are proposed, none of them provide for a substantially selective extraction of tellurium in the copper electrorefining slimes without a secondary treatment to remove interfering elements.
Although the copper smelting process used for producing copper anodes is substantially similar throughout the world, it is common knowledge that copper anode compositions vary because the composition of the copper concentrate used as the starting material in the anode furnace is also variable. The slimes produced from copper electrorefining will therefore also have various compositions.
It would therefore be highly desirable to develop a method for the substantially selective extraction of tellurium from tellurium-containing materials such as copper electrorefining slimes. Such method should be versatile enough so that it could be easily implemented, no matter what the chemical composition of the slimes is.